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Gauge calculator
Crochet a swatch, count the stitches across 4 inches, and put both numbers in below.
Go up a hook size.
Your stitches are smallerthan the pattern's (18 vs 16 per 4 in), so a bigger hook brings you closer. Swatch again after changing.
If you'd rather keep your hook and adjust the count instead:
135 stitches
instead of 120. Only do this on a pattern where the stitch count is flexible — anything with a stitch repeat or shaping needs the hook change instead.
Working 120 stitches at your gauge gives you 26.7 in (67.7 cm) where the pattern intends 30.0 in (76.2 cm).
Why gauge matters
Gauge is how many stitches you fit into a set width. Two people can work the same pattern with the same hook and the same yarn and end up with garments a size apart, because tension is personal. A swatch takes ten minutes; a jumper that comes out two inches too small takes a weekend.
The rule people get backwards: if you fit more stitches into 4 inches than the pattern does, your stitches are smaller, so you need a bigger hook — not a smaller one.